Well ar.com is itself a domain name so are they actually selling domains or subdomains? Seems like the latter to me. I would think it's not a real registration, but a piggy back. Just like forums.devshed.com is a subdomain of devshed.com. You'd pretty much be at the mercy of whoever owns ar.com to keep pointing to your server. This may be an "alternative" but it's not a domain?

From what I gather they are just different top level domains, not sub-domains.

Just like the .co, .biz, .info, .me, .uk.com, etc.

I have seen dozens of these advertised, but they have never really caught on because there are so many domain names already, and anyone with somedomain.info should worry because visitors will automatically type in somedomain.com.